1. Field of the Invention
The present invention relates generally to an apparatus and method for improving a success rate of data transmission in a mobile communication system. In particular, the present invention relates to an apparatus and method for improving reception efficiency of packet data control information in a mobile communication system supporting packet data transmission.
2. Description of the Related Art
Mobile communication systems are provided to allow mobile stations to perform communication regardless of their locations. A Code Division Multiple Access (CDMA) system is a typical example of the mobile communication system. The CDMA system, a synchronous mobile communication system, has been evolving from an IS-95 system into IS-2000 system, CDMA2000 1x Evolution for Data Only (1x EV-DO) system, and CDMA2000 1x Evolution for Data & Voice (1x EV-DV) system. Currently, as for the synchronous mobile communication system, standardization on the CDMA2000 1x EV-DV system has entered into its final phase.
All the systems stated above can support packet data transmission, and the CDMA2000 1x EV-DO system and the CDMA2000 1x EV-DV system can support high-speed packet data transmission. The two systems support high-speed packet data transmission using different schemes. First, a description will be made of packet data transmission in the CDMA2000 1x EV-DV system, the most advanced mobile communication system.
FIG. 1 is a block diagram for a packet data service in a CDMA2000 1x EV-DV system based on the current standard. As illustrated in FIG. 1, a base station (BS) 100 has a plurality of mobile stations (MSs) 111, 112 and 113. To transmit forward high-speed packet data to a particular mobile station, the base station 100 transmits the data over a forward packet data channel (F-PDCH). In order to transmit data over the high-speed packet data channel, the base station 100 should necessarily transmit a forward packet data control channel (F-PDCCH) with the F-PDCH. That is, according to the standard, the F-PDCCH has the same transmission duration and transmission instant as the F-PDCH for carrying a transmission packet. Thus, two types of data on a packet data channel and a packet data control channel are simultaneously transmitted to a mobile station. Therefore, the packet data control channel is a physical channel that the base station 100 should necessarily transmit in order to transmit a packet data service to a particular mobile station. Information transmitted over the packet data control channel includes:
1) Walsh_Mask: information on a fragmented Walsh code available for a forward packet data channel (F-PDCH) at stated periods.
2) MAC_ID (Medium Access Control layer Identification): MAC ID of a mobile station (MS) to which F-PDCH is assigned.
3) ACID (ARQ (Automatic Repeat Request) Channel ID): ID for identifying 4 ARQ channels.
4) SPID (Subpacket ID): ID for identifying an IR pattern of a subpacket.
5) EP_NEW: information for identifying two consecutive encoder packets in the same ARQ channel.
6) EP_SIZE: size (or number) of bits constituting an encoder packet.
7) LWCI (Last Walsh Code Index): information on a Walsh code used for F-PDCH.
In the CDMA2000 1x EV-DV system, a forward packet data control channel has 3 types of slot formats: 1-slot format, 2-slot format and 4-slot format, and each slot is 1.25 msec long. Therefore, the 1-slot format, 2-slot format and 4-slot format have transmission durations of 1.25 msec, 2.5 msec and 5.0 msec. respectively.
When the base station 100 transmits packet data to a mobile station, a target mobile station to which the packet data is to be transmitted is selected by a scheduler (not shown in FIG. 1) included in the base station 100. The scheduler in the base station 100 selects a target mobile station to which it will transmit packet data at every transmission instant by considering channel information and a status of a buffer in which the transmission data is stored. The channel information includes a carrier-to-noise ratio (CNR) or a carrier-to-interference ratio (CIR). After selecting a particular target mobile station by considering the channel information and the buffer status information, the scheduler in the base station 100 also determines the number of slots for which it will transmit packet data. In this case, however, the base station does not transmit slot format information (SFI) of a forward packet data control channel (F-PDCCH), determined by the base station 100, to the target mobile station receiving the packet data. Therefore, an F-PDCCH receiver of the mobile station must detect slot format information (SFI) determined by the base station 100 from a received F-PDCCH signal. Such a slot format detection scheme in which an F-PDCCH receiver of a mobile station detects a slot format is called “Blind Slot Format Detection (BSFD).”
FIG. 2 is a block diagram of a transmitter for transmitting data on a forward packet data control channel and a control message on the packet data control channel based on the 1x EV-DV standard. In FIG. 2, 1-slot format, 2-slot format and 4-slot format are represented by n=1, n=2 and n=4, respectively. Different symbol repetition and symbol puncturing are selectively used according to the slot formats. A description will now be made of information transmitted in FIG. 2, and a structure and operation for processing the information.
A control message 201 transmitted over a forward packet data control channel comprises the information described above, and the control message 201 comprises 13 bits. In FIG. 2, an expression of “1.25n” refers to the product of a unit slot length of 1.25 msec and a slot format value of ‘n’. The 13-bit control message 201 is input to an adder 211. In addition, because the CDMA2000 1x EV-DV system is a synchronous system, a system time 202 matched to a reference time is input to an offset selector 210. The system time is used to randomize information bits transmitted over a forward packet data control channel and convert the randomized information bits into a random sequence. Therefore, a 13-bit random number is received from the system time every 1.25 msec. Accordingly, the offset selector 210 generates an offset to be used in the base station using the received system time, and outputs the offset to the adder 211. The adder 211 adds the received control message 201 to the offset in synchronism with the system time, and outputs the addition result to a Medium Access Control layer Identification (MAC_ID) combiner 212.
The MAC_ID combiner 212 receives an 8-bit MAC_ID 203 for identifying users. The MAC_ID combiner 212 exclusive-ORs (XORs) the received control message and the 8-bit MAC_ID 203 according to a particular binary pattern. XORing the control message and the MAC_ID 203 in the MAC_ID combiner 212 is performed because double CRCs are used. The double CRCs can be classified into an “outer frame quality indicator” and an “inner frame quality indicator.” The outer frame quality indicator is XORed with the MAC_ID. Therefore, in FIG. 2, the MAC_ID combiner 212 is represented by an “8-bit CRC-covered MAC_ID.”
Information output from the MAC_ID combiner 212 is input to a CRC adder 213. The CRC adder 213 adds an 8-bit CRC to the information output from the MAC_ID combiner 212 so that a receiver can determine whether a received control message is defective. The output of the CRC adder 213 becomes an inner frame quality indicator. Information output from the CRC adder 213 is input to a tail bit adder 214. The tail bit adder 214 adds 8 tail bits to the CRC-added information. The tail bits are used for zero state termination performed in a convolutional encoder 215. That is, if a 13-bit information word to which the MAC_ID and CRC are added are input together with 8 tail bits, a convolutional code always terminates at a zero state on a trellis. Information output from the tail bit adder 214 is input to the convolutional encoder 215. The convolutional encoder 215 performs encoding for correcting an error in a transmission control message from noises occurring in a radio environment of a forward packet data control channel. A coding rate is set differently according to the slot format.
An output of the convolutional encoder 215 undergoes symbol repetition in a symbol repeater 216, and undergoes symbol puncturing in a symbol puncturer 217, and an output of the symbol puncturer 217 is input to a block interleaver 218. In the symbol repeater 216 and the symbol puncturer 217, symbol repetition and symbol puncturing are also performed differently according to slot format, as shown in the bottom of FIG. 2. The block interleaver 218 block-interleaves input symbols according to the slot format, and the block-interleaved symbols undergo signal mapping in a signal point mapper 219. The mapped symbols after being block-interleaved are multiplied by a channel gain in a channel gainer 220, and then transmitted over a forward packet data control channel.
A description will now be made of a structure of a receiver for receiving a forward packet data control channel and a method for checking performance of the receiver in a CDMA2000 1x EV-DV system using the forward packet data control channel. FIG. 3 is a simplified block diagram of a forward packet data control channel receiver for receiving information on a forward packet data control channel in a CDMA2000 1x EV-DV system.
Referring to FIG. 3, a transmission control message, or data, is input to a double CRC adder 301, and the CRC adder 301 performs double CRC processing on the received control message using MAC_ID and CRC added thereto. The double CRC-processed data is coded in a convolutional encoder 302. The coded symbols are subjected to symbol repetition and symbol puncturing in a symbol repeating and puncturing part 303, and then subjected to channel interleaving in a channel interleaver 304. The channel interleaver 304 is used to scatter burst errors occurring in a received signal due to multipath fading channel. The symbols interleaved by the channel interleaver 304 are input to a receiver through a channel environment 310.
The receiver is roughly divided into a reception processor 320 and a blind detector 330. A description will first be made of the reception processor 320. A channel deinterleaver 321 deinterleaves channel-interleaved symbols. The deinterleaved symbols are input to a symbol combining and erasure insertion part 322. The symbol combining and erasure insertion part 322 performs a reverse process of the symbol repetition and symbol puncturing process performed for transmission of a forward packet data control channel, on the deinterleaved symbols. The symbols output from the symbol combining and erasure insertion part 322 are input to a Viterbi decoder 323. The Viterbi decoder 323 is a general decoder used for decoding the symbols convolutional-coded by the convolutional encoder 302. The Viterbi decoder 323 decodes the convolution-coded symbols and outputs a control message. A CRC/MAC_ID checker 324 checks CRC and MAC_ID in the control message. A method detecting a control message on a forward packet data control channel in the CRC/MAC_ID checker 324 can be roughly divided into the following two methods.
In a first method, a receiver performs CRC check using both a Viterbi-decoded 13-bit information word and an inner CRC coded with MAC_ID, and then detects a control message therefrom. In a second method, the receiver additionally performs outer CRC check, or actual CRC check, after performing the first CRC check, and detects a control message only when the two CRC check results are both good.
A detailed description will now be made of 7 types of control messages transmitted over the packet data control channel. As illustrated in FIG. 4A, a message transmitted over the packet data control channel can be roughly divided into two parts. FIG. 4A is a diagram illustrating a format of a control message transmitted over a packet data control channel. As illustrated in FIG. 4A, a control message transmitted over a packet data control channel is roughly divided into a MAC_ID part 410 and a service data unit (SDU) part 420. The MAC_ID part 410 comprises 8 bits and the SDU part 420 comprises 13 bits, so that the packet data control channel receives information of a total of 21 bits. To indicate that packet data is transmitted over a packet data channel, the SDU part 420 indicates a control message on a packet data control channel is configured as illustrated in FIG. 4B. To transmit information indicating early termination of cell switching or conversion to an activated mode, the control message is configured as illustrated in 4C. Finally, to transmit information on a Walsh mask available for all mobile stations located in a base station to the mobile stations, the control message on a packet data control channel is configured as illustrated in FIG. 4D.
As illustrated in FIG. 4D, when information on a Walsh mask available for mobile stations is transmitted, the MAC_ID part 410 comprises all zero bits. In this case, a Walsh mask to be used for all mobile stations in communication with a corresponding base station must be changed. Therefore, the mobile station always checks the MAC_ID when decoding a forward packet data control channel, and performs different operations according to whether the MAC_ID has all zero bits.
For high-speed data transmission, the CDMA2000 1x EV-DV system employs Fast Hybrid Automatic Repeat Request (FHARQ) in order to improve the performance of a physical channel. Commonly, FHARQ uses N ARQ channels, and the CDMA2000 1x EV-DV system employs N=4 FHARQ. With reference to FIGS. 5A and 5B, an example of N=4 FHARQ will be described herein below.
FIG. 5A is a timing diagram illustrating transmission of packet data and ACK/NAK signal in a CDMA2000 1x EV-DV system employing N=4 FHARQ in which packet data is continuously transmitted to mobile stations.
As illustrated in FIG. 5A, a base station, or a transmitter, can continuously transmit data through a maximum of 4 HARQ channels. Therefore, in the case where the base station continuously performs HARQ transmission to 4 mobile stations A, B, C and D, the mobile station A is assigned HARQ ID=0, the mobile station B is assigned HARQ ID=1, the mobile station C is assigned HARQ ID=2, the mobile station D is assigned HARQ ID=3. Thereafter, HARQ ID=0 can be reassigned to the mobile station A, or to another mobile station. In the case of FIGS. 5A and 5B, HARQ ID=0 is reassigned to the mobile station A. A transmission scheme in which FHARQ channels are assigned to different users in this manner is called “user diversity.” User diversity has been proposed to maximize the efficiency of channel resources.
Referring to FIG. 5A, packet data 510a to be transmitted to the mobile station A is transmitted over a forward packet data channel 511 and information on the packet data 510a is transmitted over a forward packet data control channel 512. Then a receiver, or the mobile station A, receives packet data 510b that experienced a change in radio channel environment. Thereafter, the receiver has a no-operation interval (NOI) 502a for which it receives no signal over a packet data channel and a packet data control channel until an FHAQR channel is assigned again thereto. For the NOI 502a, the receiver performs demodulation and decoding on the received packet data, and transmits a response signal, or ACK/NAK signal, over a reverse ACK channel (R-ACKCH). Then the base station transmits new packet data if an ACK signal is received from the mobile station A, and retransmits the initially-transmitted packet data if a NAK signal is received from the mobile station A. The packet data to be initially transmitted or to be retransmitted is represented by reference numeral 520a, and the packet data that experienced a change in radio channel environment is represented by reference numeral 520b. 
With reference to FIG. 5A, a description has been made of the NOI 502a for which data is transmitted to other mobile stations. As another example, there is a no-operation interval for which all mobile stations are inactivated as no data is transmitted from the base station to the mobile stations. A description thereof will be made with reference to FIG. 5B.
FIG. 5B is a timing diagram illustrating transmission of packet data and ACK/NAK signal in a CDMA2000 1x EV-DV system employing N=4 FHARQ in which there is an interval for which no packet data is transmitted. Referring to FIG. 5B, packet data 510a to be transmitted from the base station to the mobile station A is transmitted over a forward packet data channel 511 and information on the packet data 510a is transmitted over a forward packet data control channel 512. Then a receiver, or the mobile station A, receives packet data 510b that experienced a change in radio channel environment. Thereafter, the receiver has a no-operation interval (NOI) 502b for which it receives no signal over a packet data channel and a packet data control channel until a FHAQR channel is assigned again thereto. The NOI 502b is different from the NOI 502a. The base station transmits packet data to other mobile station for the NOI 502a, but the base station transmits packet data to none of the mobile stations for the NOI 502b. To distinguish the NOIs from each other, the NOI 502a will be refereed to as a “transmission NOI,” and the NOI 502b will be referred to as a “non-transmission NOI.” For the non-transmission NOI, no data is transmitted and only noises are transmitted. Thus, for both the non-transmission NOI and the transmission NOI, the mobile station A is not assigned a packet data control channel, so that it should not perform any operation.
Referring back to FIG. 5B, the receiver receiving the packet data performs demodulation and decoding on the received packet data, and transmits an ACK/NAK signal over a reverse ACK channel (R-ACKCH). Then the base station transmits new packet data if an ACK signal is received from the mobile station A, and retransmits the initially-transmitted packet data if a NAK signal is received from the mobile station A. In the case of FIG. 5B, the mobile station A retransmits an ACK/NAK signal 514 as the base station fails to receive an initially-transmitted ACK/NAK signal 513. The packet data to be initially transmitted or to be retransmitted in response to the retransmitted ACK/NAK signal 514 is represented by reference numeral 520a, and the packet data that experienced a change in radio channel environment is represented by reference numeral 520b. 
In other case, all FHARQ channels may be assigned to only one mobile station. However, a description thereof will be omitted herein.
According to the CDMA2000 1x EV-DV standard, a mobile station using a packet data channel for packet transmission demodulates packet data received over the packet data channel only when a packet data control channel is assigned thereto. Based on the demodulation result, the mobile station transmits an ACK/NAK signal over a reverse ACK channel. In an actual operation of the system, however, a mobile station may possibly make an error due to noises and disturbances occurring in a channel. The mobile station makes an error in the following cases.
First, although a base station transmits packet data and a packet data control message to a particular mobile station, the mobile station may fail to correctly receive the packet data control message due to noises or disturbances in a packet data control channel. In this case, due to an error in the packet data control channel, the mobile station cannot recognize whether a packet data channel is transmitted. Therefore, the mobile station fails to receive packet data transmitted by the base station. Although the mobile station receives packet data over a packet data channel, it fails in decoding the packet data received over the packet data channel due to a defective control message. In this case, the mobile station transmits a NAK signal over a reverse ACK channel. However, because the packet data can be retransmitted by FHARQ, the mobile station does not have a serious problem except a slight delay and a reduction in transmission efficiency of channels.
Second, although a base station transmits packet data and a packet data control message to a particular mobile station, the mobile station may fail to correctly receive the packet data and the packet data control message due to noises or disturbances in a packet data control channel. In particular, the MAC_ID has all zero bits as illustrated in FIG. 4D due to an error in the packet data control channel. In this case, because the MAC_ID has all zero bits, the mobile station mistakes the packet data control message for Walsh mask update information. Therefore, the mobile station changes its own Walsh mask due to the wrong information. Thereafter, although the base station transmits packet data, the mobile station cannot decode packet data received over a packet data channel due to a Walsh decoding error. This continues until the Walsh mask is updated. Therefore, when the base station transmits packet data to the mobile station whose Walsh mask is updated due to an error, the mobile station cannot continuously receive packet data, thereby interrupting a packet data service.
This will be described with reference to FIG. 6 by way of example. FIG. 6 is a timing diagram illustrating a time for which a mobile station fails to receive packet data as a Walsh mask is changed due to an error in a forward packet data control channel. Referring to FIG. 6, a Walsh mask currently in use is transmitted at a time T0. A base station configures a Walsh mask into a control message illustrated in FIG. 4D, and transmits the control message over a packet data control channel at stated periods. In FIG. 6, a Walsh mask update period 600 ranges from the time T0 to a time T2. Therefore, the receiver should continuously use the Walsh mask received at the time T0 until a time at which it receives the next Walsh mask. However, in the second case, the mobile station cannot continuously receive packet data from a time T1 at which a Walsh mask is changed due to a Walsh mask error (See 602) to the time T2 at which the Walsh mask is updated.
If the Walsh mask is updated in this way, even though the receiver determines from a packet data control channel that packet data is transmitted thereto, it fails in demodulating and decoding the packet data. Therefore, the receiver continuously transmits a NAK signal over a reverse ACK channel each time packet data is received. Such a false NAK signal is called a “false alarm.”
Third, when a base station transmits packet data and a packet data control message to a particular mobile station, a mobile station may mistake the packet data control message for its packet data control message due to noises or disturbances in the packet data control channel. In this case, the mobile station fails in decoding packet data received over a forward packet data channel, so that it cannot extract normal data. Therefore, the mobile station transmits a NAK signal over a reverse ACK channel. In this case, however, because the base station transmitted no packet data to the mobile station, the base station is allowed to disregard the NAK signal received over the reverse ACK channel. Alternatively, the mobile station can check again the control message which was not transmitted thereto through MAC ID detection. Therefore, in this case, the mobile station does not have a serious problem. However, occupation of a reverse ACK channel (R-ACKCH) for reverse transmission of an ACK/NAK signal and a reverse channel quality indicator channel (R-CQICH) for CIR transmission by a non-selected mobile station causes unnecessary occupation of reverse channel resources and interference to R-ACKCH of a normal mobile station, thereby deteriorating the quality of an R-ACKCH signal from the selected mobile station.
Fourth, when a base station transmits packet data and a packet data control message to a particular mobile station, a mobile station may mistake the packet data, control message for its packet data control message due to noises or disturbances in the packet data control channel and, particularly, mistakes MAC_ID for all-zero MAC_ID, i.e., Walsh mask update information, due to an error in the forward packet data control channel. In this case, the mobile station changes its own Walsh mask due to the incorrect information. Therefore, although the mobile station decodes packet data received over the forward packet data channel, most of the packet data suffers from decoding error because of a Walsh demodulation error. Thus, the mobile station transmits a NAK signal over a reverse ACK channel. As described in the second case, such an event is continuously repeated unless the Walsh mask is updated again.
As described above, when the Walsh mask is changed due to an error in a packet data control channel, an error continuously occurs in received packet data. Therefore, unless the base station transmits again a Walsh mask, a reception error for a forward packet data channel continuously occurs due to the wrong Walsh mask information. Such an event can happen in the case of FIGS. 5A and 5B. Therefore, the receiver, or the mobile station, requires a method for preventing such an error.